In Japanese Patent Publication No. H10-203338, a brake-by-wire vehicle brake control device is proposed which has four pumps respectively for the four wheels of a vehicle. Two of the four pumps are located in a common conduit system for two of the four wheels and are driven by a common electrical motor. The other two of the four pumps are located in another common conduit system for the other two of the four wheels and are driven by another common electrical motor.
The above vehicle brake control device uses each of the electrical motors to drive two pumps in a conduit system. In executing an anti-lock brake (hereinafter referred to as ABS) control, optimum rotational speeds for the electrical motors depend on a friction coefficient μ of a road on which the vehicle is traveling. Therefore, it is desirable to determine target rotational speeds of the electrical motors based on the friction coefficient μ.
FIG. 11 includes graphs showing a relation between a W/C pressure increase rate and a target rotational speed of an electrical motor in a pressure increase mode of the ABS control and a relation between a W/C pressure decrease rate and a target rotational speed of the electrical motor in a pressure decrease mode of the ABS control. In each of the graphs, the relation covers both situations that the vehicle is traveling on a road with a high friction coefficient μ and that the vehicle is traveling on a road with a low friction coefficient μ. Hereinafter, the road with a high friction coefficient μ and the road with a low friction coefficient μ are referred to as a high friction road and a low friction road, respectively.
As shown in the drawing, a high pressure increase rate is required at the high friction road. Therefore, it is desired at the high friction road that the electrical motor achieves a high rotational speed which is equal to or more than a rotational speed Na1 in FIG. 11 and accordingly satisfies the required pressure increase rate. On the other hand, a high pressure decrease rate is required at the high friction road because it is necessary to decrease the W/C pressure rapidly from a high pressure when a slip ratio of the corresponding wheel is large. The high pressure decrease rate puts an upper limit (a rotational speed Na2 in FIG. 11) to the rotational speed of the electrical motor. It is therefore desirable to adjust the rotational speed of the electrical motor at the high friction road so that the adjusted rotational speed is within a range between the rotational speed Na1 and the rotational speed Na2.
At the low friction road, the pressure increase speed is not required to be as high as at the high friction road. Therefore, it is desirable that the rotational speed of the electrical motor is equal to or more than a rotational speed Nb1 in FIG. 11 and accordingly satisfies the lower pressure increase rate. When the slip ratio is high at the low friction road, the W/C pressure is decreased from a lower pressure. However, the high pressure decrease rate is still required because the decrease rate of the W/C pressure should be high enough. The high pressure decrease rate puts an upper limit (a rotational speed Nb2 in FIG. 11) to the rotational speed of the electrical motor. It is therefore desirable to adjust the rotational speed of the electrical motor at the high friction road so that the adjusted rotational speed is within a range between the rotational speed Nb1 and the rotational speed Nb2.
However, if the vehicle is traveling on the sudden change road in which the friction coefficient drastically changes from a higher value to a lower value, it is impossible to change the rotational speeds of the electrical motors immediately from the rotational speeds suitable for the high friction road to the rotational speeds suitable for the low friction road, because the rotational speeds suitable for the high friction road significantly differ from the rotational speeds suitable for the low friction road. Therefore, a sufficient deceleration of the vehicle cannot be obtained in the ABS control and a suitable ABS control is accordingly difficult to achieve.